This invention relates generally to pulse amplifying and shaping circuits and more particularly to an improved circuit of this kind for use as a gate driver for a periodically triggered thyristor.
Many electronic circuits and apparatus employ solid-state controllable switching devices known as thyristors or silicon controlled rectifiers (SCRs). A thyristor is typically a three-electrode device having an anode, a cathode, and a control or gate terminal. When its anode and cathode are externally connected in series with an electric power load and a source of forward anode voltage (i.e., anode potential is positive with respect to cathode), a thyristor will ordinarily block appreciable load current until a firing signal of appropriate amplitude and duration is applied to the control terminal, whereupon it switches from its blocking or "off" state to a conducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. To generate the firing signal required to trigger a high-power thyristor, it is common practice to use a gate driver that is activated by a command or gating signal which in turn is supplied by associated control means. The criteria for designing a gate driver are well known in the art-see, for example, the chapter entitled "Gate Trigger Characteristics, Ratings, and Methods" on pages 71-122 of the SCR Manual (Fifth Ed., 1972 ) published by the General Electric Company (Electronics Park, Syracuse, N.Y.). Generally speaking the waveform of the firing signal should be characterized by a sharply rising front, a high amplitude, and a sufficient length (duration) to ensure successful turn on of the thyristor when commanded by the control means.
In many practical applications a thyristor is triggered by a single pulse-like firing signal having a very fast rise time, a high amplitude, and a relatively short duration (of the order of 25 microseconds or less). When only a single pulse is required, it is not difficult to design a gate driver for such applications. For example, in a known gate driver of this type a precharged capacitor is discharged through the primary winding of a pulse transformer so that the secondary current (which is the firing signal) rises abruptly to a relatively high peak, and a series LC circuit is connected across the capacitor so that its subsequent discharge will slightly broaden this pulse of current. In other applications, a thyristor requires a firing signal having a much longer duration (e.g., longer than 1 millisecond). In this case the initial rise time of the firing signal is not critical and can be relatively slow, and gate drivers using long pulse width transformers or known filtering techniques to prolong or sustain the firing signal can therefore be applied.
In some cases a combination of short single pulse and long sustained firing signals is needed. An example of this requirement is found in co-pending U.S. patent application Ser. No. 035,352 filed concurrently herewith for R. B. Bailey and T. D. Stitt and assigned to the General Electric Company, which application discloses a chopper type electric propulsion system for d-c traction motors. For reasons that are explained in the referenced patent application, in normal operation the main thyristor of the chopper is periodically fired in response to gating signals of relatively short duration (approximately 10 microseconds) that recur at either a constant frequency (300 Hz) or a variable frequency that is lower than the constant frequency, but to turn on the chopper at the beginning of an electrical braking mode of operation of the propulsion system, the main thyristor is initially triggered by an extended firing signal generated in response to a 2-millisecond burst of high frequency (e.g., 21.6 KHz) discrete pulses (10 to 20 microseconds each). In this case the use of prior art techniques in the gate driver to obtain the extended firing signal is not practical because the resulting retardation of rise time would be unsatisfactory in normal operation. Other known pulse shaping circuits that can generate extended firing signals are not ideal for dual-purpose gate drivers because of one or more of the following shortcomings: electrical isolation between control and power circuits is insufficient, an a-c control power source is required, cost is too high, or initial rise time is too slow.